Granulocyte colony-stimulating factor (G-CSF), alone or in combination with dexamethasone, is becoming a standard mobilizing regimen for granulocyte donors. The effect of these agents on donor granulocyte counts is well known, but effects on other blood elements are not well described. In a prospective blinded study, we collected granu-locyte concentrates from six donors on three occasions each. In random order, the donors received either dexamethasone (8 mg PO), G-CSF (5 ug/kg SQ), or both on day 1. On day 2, granulocyte concentrates were collected by processing 7 liters on a CS3000 blood cell separator. Donor granulocyte counts returned to baseline after 2 days in all three types of collections and granulocytopenia did not occur. Donor platelet counts were similar using all three mobilization methods and returned to normal after 1 week. When dexamethasone was given, potassium, phosphorus, and uric acid levels fell, but BUN increased. When G-CSF was given, potassium levels fell and leukocyte dehydrogenase (LDH) increased. When both were given, potassium and phosphorus levels decreased, but BUN and LDH increased. Alkaline phosphatase levels were not affected by any of the three mobilization regimens. All chemistries returned to baseline 1 week following the collection. In conclusion, mobilization of granulocytes with either G-CSF or dexamethasone is associated with mild changes in blood chemistries and a mild transient fall in platelet counts. Blood chemistries and platelet counts returned to baseline levels promptly. It is unclear if these changes have any clinical consequences, but they suggest that mobilized collections should not be performed at greater than one week intervals. A comparison of symptoms in donors given G-CSF or dexametha-sone is ongoing. Additional studies will compare different methods to administer G-CSF.